Taking out heterodyning from multi-channel LVDT measurement systems

December 27, 2016 // By John Matlack
With its DIN-rail-mounting smart LVDT signal conditioner module S1A, H. G. Schaevitz LLC, Alliance Sensors Group has solved the age old problem of cross talk between sensors even if the master signal conditioner fails. When LVDT signal cables are bundled together or laid close to each other in a wire trough, even for short distances, the result can be a mutual interference phenomenon called heterodyning or “beating”.

This effect is created when the frequencies of the oscillator in the individual signal conditioner driving the excitation signal to each LVDT vary slightly. Since the signal cables are close together, they can capacitively couple among each other, resulting in a very low frequency signal that is the difference between the frequencies of the individual oscillators.

This difference frequency signal can ride on the DC output of the LVDT signal conditioners, appearing as low frequency ripple or noise with a period measured in fractions of a second, or as a repetitive slow drift with a period of many seconds.

When a pair of LVDTs are used together for redundancy, this effect may be evident on only one channel of the system. In systems with larger numbers of LVDT channels, this effect may be found in one or more channels, depending on many factors like cable length and layout as well as shielding and grounding.

The chart below show an example of this drifting over time on the left side and the elimination of the effect utilizing the Master /Slave function

Graph is courtesy of Rawhide Energy Station, Platte River Power Authority, Fort Collins CO.

The waveform above in the output was noted in right reheat and intercept valve positioners that was using dual LVDTs in redundant systems. The master / slave function removed the beat frequency signal and the waveform flattened.  

Typically the way to solve this problem in multiple channel systems is to create a “master” oscillator by using the oscillator from one of the signal conditioners to supply the same excitation frequency to all of the LVDTs. The oscillators in the other LVDT signal conditioners are overridden by the master and considered “slaves”.